1. FIELD OF THE INVENTION
This invention relates to a gas-generating composition useful for inflating air bags in motor vehicles, when a collision occurs, to cushion vehicle occupants against forcible contact with hard surfaces of the vehicle body.
2. DESCRIPTION OF THE PRIOR ART
Recently automobile accidents have been greatly increasing year-by-year, so that the development of inflatable air bags for protecting the human body is strongly desired in order to protect the occupants of automobiles from serious injury in the event of a collision. Some types of such bags are in practical use. Available gas-generating sources for such air bags include liquefied gases, solid gas-generating agents, and combinations of them. However, the use of solid gas-generating agents is preferred because such agents can be contained in a small device.
The gas for expanding the air bag must be completely nontoxic, because it might possibly contact directly with the automobile occupants if it should leak from the bag. Therefore, mixtures essentially composed of alkali metal and alkaline earth metal salts of hydrazoic acid (hereinafter designated as metal azides) are particularly preferred, as described in U.S. Pat. No. 2,981,616, because they decompose to generate clean gases. However, many residues of alkali metals, alkaline earth metals, and their oxides (hereinafter designated as residues), which are produced simultaneously with the inflating gas, are chemically active and toxic. Some of them react vigorously with moisture in the air to evolve hydrogen, thus increasing the danger of fire. Moreover, if the residues are inhaled by humans along with the gas leaked from the air bag, or if they should contact human skin, they would seriously injure the human body.
Thus, in developing gas-generating agents mainly consisting of these metal azides, there must also be considered the use of a chemically detoxicating composition or a filtering system to readily capture these residues.
The gas-generating agents mainly composed of a metal azide known in the prior art are mixtures of (A) an alkali metal azide and/or an alkaline earth metal azide and (B) an oxidizing agent (hereinafter designated as oxidant) of (1) one or more nitrates and perchlorates of alkali metals and alkaline earth metals and (2) one or more of metal oxides.
The theoretical gas-generating mechanisms of an agent comprising a metal azide and an oxidant are as follows: EQU 10 NaN.sub.3 + 2 KNO.sub.3 .fwdarw. 5 Na.sub.2 O + K.sub.2 O + 16 N.sub.2 ( 1) EQU 8 naN.sub.3 + KClO.sub.4 .fwdarw. 4 Na.sub.2 O + KCl + 12 N.sub.2 ( 2)
however, when the theoretical amounts of sodium azide and potassium nitrate in reaction (1) are merely mechanically mixed or the theoretical amounts of sodium azide and potassium perchlorate in reaction (2) are merely mechanically mixed, and further when such compositions are molded into shapes suitable for combustion, such moldings always produced sodium metal and sodium peroxide during combustion.
This is ascribed to the belief that the gas-generating agent composed of the metal azide and oxidant in the prior art actually reacts as follows: EQU 10 NaN.sub.3 + 2 KNO.sub.3 .fwdarw. a Na + b Na.sub. 2 O + c Na.sub.2 O.sub.2 + 16 N.sub.2 + K.sub.2 O (3) EQU wherein a + 2(b+c) = 10, EQU 8 NaN.sub.3 + KClO.sub.4 .fwdarw. x Na.sub.2 O.sub.2 + y Na.sub.2 O + z Na + KCl + 12 N.sub.2 ( 4) EQU wherein O&lt;x.ltoreq.2, O.ltoreq.y.ltoreq.4, O&lt;z.ltoreq.4, and x + y + z = 8.
The sodium metal and sodium peroxide produced by the foregoing reactions (3) and (4) are very fine toxic powders and they disperse into the generated gas. Therefore, in using the prior art gas-generating agent of the metal azide and oxidant, the air bag system must be equipped with a device for capturing the residues.
Consequently, there are already known many methods for separating the residues from the generated gas, including the use of a mechanical filtering system provided in the gas-flow path and the use of a filter layer which is provided for reacting with and fixing the residues. The mechanical filtering system includes baffle plates provided in the gas-flow path, thereby to change the direction of gas-flow and to separate the gas and residues as described in U.S. Pat. No. 2,756,375; and the filter layer combined with wire nets of different meshes as described in Japanese Patent (unexamined) No. 1974-13,838.
A chemical filtering system includes a layer packed with boric anhydride or silicone resin coated on a carrier such as alumina, thereby to capture the residues dispersed in the gas by reacting the residues with the boric anhydride or silicone resin in the layer while the gas passes through the layer as described in U.S. Pat. application Ser. No. 152,897, U.S. Pat. No. 3,797,854. However, the residues produced during combustion are very fine and they are transferred along with a lot of the generated gas. Therefore, the foregoing filtering systems cannot filter the solid residues sufficiently.
Contrary to these methods, there is also known a method for changing the residues produced from the gas-generating agent of metal azide and oxidant into a nontoxic form by using silica and silicate (hereinafter designated as silicon dioxide) as described in West German Patent (unexamined) No. 22 36 175.7. The silicon dioxide can form a low melting glass with the residues, as follows: EQU 5 Na.sub.2 O + K.sub.2 O + 6 SiO.sub.2 .fwdarw. 5 Na.sub.2 O.K.sub.2 O.6 SiO.sub.2 ( 5)
further, there have been known methods using metal oxides such as lead oxide and ferric oxide along with the silicon dioxide to promote the reaction (5). Such methods can make the gas generator structurally simpler and functionally more effective than the method employing a filtering system as discussed above.
In the method using silicon dioxide, the silicon dioxide must be mixed with the metal azide and the oxidant uniformly in an amount sufficient to react the silicon dioxide stoichiometrically with the residues as shown in reaction (5). By the conventional methods of mechanical mixing, however, it is difficult to mix enough silicon dioxide sufficiently uniformly with the metal azide and oxidant. Thus, it is difficult to react the silicon dioxide with the residues and thereby to form a low melting glass. Further, the resulting gas-generating agent is inferior in combustibility because the components are mixed nonuniformly as described earlier.